1. Field of the Invention
The present invention relates to a composite balun for connecting a balanced transmission line and an unbalanced transmission line.
2. Description of the Related Art
A multilayer chip balun has been known as a balanced-unbalanced transformer for transmission lines in a high-frequency circuit. In general, the balun includes first and second line portions constituting a pair of balanced transmission lines, third and fourth line portions constituting a single unbalanced transmission line to be electromagnetically coupled to the balanced transmission lines, a ground electrode, and a laminate that is composed by stacking a plurality of dielectric sheets. Moreover, the first and third line portions are electromagnetically coupled at portions opposing one another across the dielectric sheet to form a coupler, while the second and fourth line portions are similarly electromagnetically coupled at portions opposing one another across the dielectric sheet to form a coupler.
In the days when only one balun was built into a single chip, when incorporating a plurality of baluns into a mobile communication apparatus such as a mobile phone or a wireless LAN device, it is required to save an area of a printed circuit board for mounting a plurality of chips. This has led to a problem of increasing the size of the communication apparatus, but there has been made an attempt to solve this problem.
For example, Japanese Unexamined Patent Application Publication No. 2002-329611 discloses a multilayer composite balun with a plurality of baluns, wherein each balun is composed by stacking a pair of balanced transmission lines, an unbalanced transmission line electromagnetically coupled to the pair of balanced transmission lines through a dielectric layer, a ground electrode opposed to at least either of the balanced and unbalanced transmission lines across the dielectric layer, and a plurality of dielectric layers.
In the case where such a composite balun is disposed at a pre-stage of a power amplifier (PA) of a mobile communication apparatus to let the balun provide a differential input to the power amplifier, or in the case where the composite balun is disposed at a post-stage of a radio frequency integrated circuit (RFIC) to supply power to the radio frequency integrated circuit through the balun, it is required that a DC voltage is supplied to the power amplifier or the radio frequency integrated circuit through the composite balun. However, Japanese Unexamined Patent Application Publication No. 2002-329611 fails to disclose a configuration which meets the above requirement.
In order to supply a DC voltage to the power amplifier or the radio frequency integrated circuit through the composite balun, moreover, measures should be taken against noise entering through a power supply circuit, but Japanese Unexamined Patent Application Publication No. 2002-329611 also fails to disclose such means.